Articles made from electrically nonconductive plastic can be metallized by an electroless metallization process. In this process, the article is first cleaned and etched, then treated with a noble metal and finally metallized. The etching is typically undertaken by means of chromosulphuric acid. The etching serves to make the surface of the article receptive to the subsequent metallization, such that the surfaces of the articles are well-wetted with the respective solutions in the subsequent treatment steps and the deposited metal ultimately has sufficiently firm adhesion on the surface.
For etching, the surface of articles, for example made from acrylonitrile-butadiene-styrene copolymer (ABS copolymer), is etched using chromosulphuric acid, so as to form surface microcaverns in which metal is deposited and subsequently adheres there firmly. After the etching, the plastic is activated for the electroless metallization by means of an activator comprising a noble metal, and then metallized electrolessly. Subsequently, a thicker metal layer can also be applied electrolytically.
Etching solutions based on chromosulphuric acid, however, are toxic and should therefore be replaced as possible.
The literature describes attempts to replace etching solutions based on chromosulphuric acid with those comprising permanganate salts.
The use of permanganates in an alkaline medium for metallization of circuit boards as a carrier of electronic circuits has long been established. Since the hexavalent state (manganate) which arises in the oxidation is water-soluble and has sufficient stability under alkaline conditions, the manganate, similarly to trivalent chromium, can be oxidized electrolytically back to the original oxidizing agent, in this case the permanganate. The document DE 196 11 137 A1 describes the use of the permanganate also for metallization of other plastics as circuit board material. For the metallization of ABS plastics, a solution of alkaline permanganate has been found to be unsuitable since it was not possible in this way to obtain a reliable, sufficient adhesion strength between metal layer and plastic substrate. This adhesion strength is determined in the “peel test”. It should have at least a value of 0.4 N/mm.
EP 1 0010 52 discloses an acidic permanganate solution which is said to be suitable for use in plastic galvanization. EP 1 0010 52 does not report the adhesion strengths achievable by this pretreatment. In-house experiments have shown that the adhesion strengths are below a value of 0.4 N/mm. Moreover, the solutions described in EP 1 0010 52 are unstable. A constant quality of the metallization therefore cannot be achieved.
As an alternative to chromosulphuric acid, WO 2009/023628 A2 proposes strongly acidic solutions comprising an alkali metal permanganate salt. The solution contains about 20 g/l alkali metal permanganate salt in 40-85% by weight phosphoric acid. Such solutions form colloidal manganese(IV) species which are difficult to remove. According to WO 2009/023628 A2, the effect of the colloids even after a short time is that coating of adequate quality is no longer possible. To solve the problem, WO 2009/023628 A2 proposes using manganese(VII) sources which do not contain any alkali metal or alkaline earth metal ions. However, the preparation of such manganese(VII) sources is costly and inconvenient.
Therefore, toxic chromosulphuric acid is still being used for etching treatment of plastics.
For industrial scale application of metallization of plastic surfaces, the articles are usually fastened to racks. These are metal carrier systems which allow the simultaneous treatment of a large number of articles with the successive solutions for the individual process steps, and last steps for electrolytic deposition of one or more metal layers. The racks are generally themselves coated with plastic. Therefore, the racks in principle likewise constitute a substrate for metallization processes on plastic surfaces.
However, the additional metallization of the racks is undesirable, since the metal layers have to be removed again from the racks after the coating of the articles. This means additional cost and inconvenience for the removal, combined with additional consumption of chemicals. Moreover, the productivity of the metallization plant in this case is lower, since the racks first have to be demetallized prior to reloading with articles. If the demetallization has to take place using semi-concentrated hydrochloric acid and/or using nitric acid, vapours and aerosols are produced, and these lead to corrosion in the environment.
A further problem is that, when rack metallization occurs, it is no longer possible to achieve a defined current density in a reproducible manner because the extent of the rack coverage is usually unknown, and the exact surface area of the rack is likewise unknown. The consequence is then usually that the metal layer applied to the galvanized plastic articles is too thin.
In the case of use of chromic acid-containing etchants, this problem is much reduced. During the etching, chromic acid also penetrates into the plastic casing of the racks and diffuses back out of it during the subsequent process steps, thus preventing metallization of the rack.
Thus, if the intention is to replace toxic chromosulphuric acid for etching treatment of plastics with environmentally safe process steps, it becomes necessary to prevent unwanted metallization of the racks.
Patent DE 195 10 855 C2 describes a process for selective or partial electrolytic metallization of nonconductive materials. In this case, the simultaneous metallization of the racks is prevented by omitting treatment steps with adsorption-promoting solutions, called conditioners. However, it is emphasized that the process for metallizing nonconductive materials in DE 195 10 855 C2 is suitable only for direct metallization.